Method of and apparatus for producing high vacuum



APPLICATION FILED JAN. 8. I917.

Patented Feb. 8, 1921.

4 SHEETSSHEET 1.

iniluviiniit w. w. CRAWFORD. METHOD OF AND APPARATUS FOR PRODUCING HIGH VACUUM.

APPLICATION FILED JAN-8,1917.

Patented Feb. 8, 1921.

4 SHEETSSHEET 2.

W. W. CRAWFORD. METHOD OF AND APPARATUS FOR PRODUCING HIGH VACUUM.

' APPLICATION FILED JAN-8,1917. 1,367,865.

Patnted Feb. 8,1921.

4 SHEETSSHEET 3.

w. w. CRAWFORD.

METHOD OF AND APPARATUS FOR PRODUCING HIGH VACUUM.

APPLICATION FILED JAN.8. 19w.

1,367,865. A Patented Feb .8; 1921 ya x 0 FlQig- L Y t I v j /151 Z Y /?1 I 1 *7 ;4 [HA 'L.

C :1 1 g? FIT c 1 T UNITED STATES PATENT OFFICE.-

WILLIAII W. CRAWFORD, OF PHILADELPHIA, PENNSYLVANIA, ASSIGNOR-TO VICTOR ELECTRIC CORPORATION, OF CHICAGO, ILLINOIS, A CORPORATION OF NEW YORK unrnon or AND APPARATUS FOR rnonucme men vacuum.

To all whom it may concern:

Be. it known that I, WILLIAM W. CRAW:

.FORD, a citizen of the UnitedStates, residing in the city of Philadelphia, county of Phlladephia, and State of Pennsylvania, have invented a new amiuseful Method of and AprX-ray tubes or other electrical vacuum tubes, or any other chambers.

, My invention resides in providing proportions and conditions of operation which render a vapor aspirator or ejector structure operativein producing these extremely high vacua. My method consists in entraining the gas to be pumped in a jet of vapor, for example, mercury, cadmium, hydrocarbon oils, etc., which has an extremely high velocity and high degree of rarefaction due to its having been expanded from a high initial pressure, five to ten millimeters of mercury, or higher, to a considerably lower pressure, estimated as one-tenth to one hundredth of millimeter of mercury. I find that a jet so produced will even when surrounded by a; high vacuum preserve its form as a free jet to a sufficient extent to enable it to entrain not only the gas to be pumped, but a great proportion of the vapor diffusely turned from the hot walls of a tube which I sometimes employ surrounding'the jet, and which I call the compression passage (corresponding to the so-called diffuser of a high pressureaspirator) and will expel such gas and. vapor together into a chamber in whi: h the pressure is many times greater than the pressure'in the vacuum space. The degree of expansion is such that a specially absorptive condition of the jet is produced,

enabling the jet to entrain gas whose pressure is only a small fraction of the pressure in the jet. a

My present explanation for the fact that the et preservesits form when surrounded by a high vacuum is that in consequence of the cooling of the vapor due to its very reat expansion the relative multi-directiona veloc'ities of the individual molecules are much reduced, while all the molecules acquire a common high velocity in the direction of he j The resultant velocities of the great Specification of Letters Patent.

Patented Feb. 8, 1921.

Application filed January 8, 1917. Serial No. 141,326.

' majority of the-molecules passing a, given point are hence. nearly equal and-parallel. therefore refer to a jet of this type as a parallel or uni-directional jet.

Collisions between such approximately parallel-moving molecules are fewer and when occurring result in -practically unchanged directions of motion, so that the number of molecules which escape from the jet is very small. a

There is a sorting action among the molecules of the free jet so that those molecules having widely divergent directions and magnitudes of velocity tend to collide and be eliminated from the jet, so that the farther the jet proceeds before striking an obstacle or before entering a region of higher pressure, the greater its purity as to parallelism of molecular motion and the less its tendency to scatter. For this reason I sometimes introduce a partition or diaphragm through an orifice in which the free jet passes.

The diaphragm separates and causes to condense in a separate chamber the vapor which escapes from the free jet, and the gas to be pumped enters the jet be:

.yond the diaphragm in the region in which the jet is most completely uni-directional.

Free parallel jets of a high degree of purity have hitherto been produced under conditions of extreme rarefaction of the vapor, but it is, so far as I am aware, my discovery that it is possible to produce a parallel or uni-directional vapor jet of sufliciently high density, that is, of sufiiciently large amount of vapor per unit of volume within the vapor jet to be operative kinetically in overcoming a considerable back pressure.

The practical criterion for the recognition of a parallel jet is, as stated, the substantial absence of scattering or diverging of the vapor even when the jet is surround: ed by a very high vacuum, and the method by which the parallel jet used in my invention is produced is, as stated, the combination of a high expansive cooling and high velocity with a final pressure and densit within the jet lower than that employe hitherto in vapor aspirators and higher than that! hitherto employed in parallel jets.

From an abstract standpoint it appears that the more nearly equalandparallel the molecular velocities become, the better the Cir results, and if the velocities were absolutely equal and parallel, there would be no lateral PIGLSIII'G. Practically, however, it is found that only the degree of expansion stated is necessary. Computing b the customary method according to the aws of adiabatic expansion, it appears that in the actual jets produced, the lowest pressures are not lower than one-tenth to one-hundredth millimeter of mercury, yet in actual operation these jets remove gas from the vacuum space when the pressure there is onl one hundredth (1/100) to one ,thousandt (1/1000), or a less fraction, of the computed pressure in the jet. The customary methods of calculation may be in error when applied to such low pressure jets, but it would seem that the principal error is that due to ignoring friction, which would result in the actual velocity bein lower, and the pressure higher, than state above. i

The term presiure is here used in the ordinary sense of an actual mechanical force between adjacent portions of the vapor, due

to the impacts set .up by molecular collisions.

The fact that the pressure in the vacuum spaceis actually lower than that in the jet is further verified by tests which have been made with a continual leakage of gas into the vacuum s ace, and varying the density of the jet. hen the density is above a certain limit, the pressure in the vacuum space is about equal to thecomputed pressure in the jet, the device then operates. as an ordinary aspirator. On lowering the densit of the jet below said limit, there is a sud en and very great decrease of pressure in the vacuum space, out of all proportion to the decrease in" the computed pressure in the jet.

In conse uence of the ractical certainty that a smal but apprecia le ressure exists parallel character of the molecular motion without a large within the jet at the point 0 entrainment, the language used above in explaining the is made to admit of slight lateral and lon 'tudinal relative motion of the molecules.

the velocities can exist, and the total velocities still be in a direction which will carry themolecules away from the vacuum space and through the compression passage proportion of them striking the walls.

Inasmuch as it is difiicult to determine the pressure of the vapor in such a jet, I prefer to define the conditions in terms vof the density and the molecular weight. For example, with a mercury vapor jet transthereto, inasmuc ensity is evidently I ing;high vacua, and applies appiioxilnately to conditions which have been produced in practice, but In invention is not limited ii as considerably higher or lower values may be used.

One advantage of the parallel jet is the elimination in a practical sense of the tendency ofthe molecules to diverge and strike at considerable angles the walls of the container or passage or to enter the vacuum space, from which follows the practical result that the countercurrent of the vapor, especially froi'n'a hot compression passage, is reduced to such extent that it does not interfere with the entry into the jet of the gas to be removed or pumped. Q

-Where the vapor jet is projected through a compression passage, the angle of incidence of the vapor, in the jet with respect to the wall of the compression passage should be acute, and preferably less-than 30 degrees, and, in general, the more acute this angle the better; and the more completely uni-directional the jet, the smaller or more acute may this angle be made and the more effective the action.

As a practical consideration, not only must the molecules passin a given point move in parallel directions, ut this'dir'ection must be nearly the same 1n different adjacent parts of the jet.- The maximum angle of diver- '30. degrees).

Those molecules of the vapor striking the wall even at razing incidence are, its believed, diffuse y returned from the wall, that is, at angles distributed in all directions practicall independent of the angle of incidence. hen the angle of incidence of the jet against the wall is small, as specified, the number of such molecules which can return directly into the high vacuum space is very small compared to the number which are reentrained by the et. This is my resent explanation of the observed fact t at in my pump a blast of va or into the high vacuum space sufficiently ense to prevent the flow of gas into thejet does not occur. A very slight amount of vapor does flow back into the high vacdum'space' and condenses there at a considerable distance from the nozzle, whenever the temperature of any art of the vacuum chamber is -suflieiently ow. This is a common feature of all vacuum pumps containing a vaporizable substance, 'for example, a liquid mercury piston pump with an ice or liquid air bath on the vacuum conflow of gas from the vacuum space into the jet is brought about, despite the higher pressure in the jet, is as follows 7 Because of the parallel or uni-directional character of the vapor jet and because the stream of gas to be exhausted is also lar ely a uni-directional jet, the directions of ow of the two jets being more or less closely coincident, a molecule of gas travels with and into the vapor jet for a relatively great dis-. tance before collision with a molecule of the vapor, and further, when it does collide, cannot acquire a backward velocity; the molecule thereby becomes eifectively entrapped in the vapor. This entry of the gas into the vapor is a special and novel variety of mo-, lecular flow, which is characteristically different from the action in high pressure pumps as well as so-called difiusion or other 1 low pressure pumps, molecular flow being characterized by the substantial disappearance of collisions between molecules of the entering gas and the vapor. This special variety of molecular flow which I call parallel molecular How, is characterized by the fact that it will occur at higher densities of the medium than ordinary or multi-direo tional molecular flow.

This explanation is in accord with the well-known fact that the pressure. exerted between two bodies of elastic fluid in contact under any conditions whatsoever is not 10- calized in the surface forming the assumed mutual boundary. The pressure is due to the collisions of the molecules of one body with thoseof the other body. It is exceptional for two molecules to meet exactly in the boundary surface. The collisions, and

the pressure, are distributed throughout a layer of appreciable depth on both sides of the surface.

Consequently, the pressure of the vapor within the jet cannot act on the gas from the vacuum space until the gas has penetrated an appreciable distance into the vapor. When this has occurred, the high velocity of the jet comes into play to sweep at least partof the gas molecules-along with the jet.

L that there is an imbalanced V This theory necessitates the supposition ressure in-the outer layer of the jet, the ten ency of which would be to alter'the motion of the vapor molecules in the outer layer, and drive them outward, or even toward the vacuum space. In view of the low pressure in the jet,-and the high velocity, it appears that a given body of vapor is not in contact with the vacuum space long enough for this alteration of motion to take place to any great extent.

It is further characteristic of my metho and apparatus that artificial cooling, particularly at the region of entry of gas into the vapor, is unnecessary, and in a practical sense inconvenientand undesirable, since it does not sensibly improve the speed or attainable vacuum of the pump.

The result, produced by my method that the pressure in the vacuum space isfar lower than andof a different order of magnitude from the lowest pressure in the vapor stream is tangibly different from that in the high pressure vapor aspirators producing partial vacua, in which, as far as I am aware, the lowest pressure produced is '.11ttle,if any, lower than the lowest pressure in the jet. My method in producin this desirable result differs from the metho of the high pressure aspirator in that I expand the vapor until a condition of arallel molecular flow, as described and defined, sets in, which requires expansion to a pressure and density within the stream of a lower order than is em loyed in high pressure vapor aspirators. y difference in method and apparatus from other high vacuum pumps consists in the fact that inmy pump the entrainment of the gas into the body of the jet is brought about by the absorptive condition of the jet, whereas in the other methods and apparatus, special structural means of entrainment are employed. 4

In the so-called condensation type of pump, the gas becomes entrapped or intermixed with the vapor at and in the region face, whereas by my method the gas enters the freejet and is elevated to the interme- I diate pressure prior to the condensation of an appreciable" portion of the ,vapor. Condensation is in my invention solely a means of enabling the medium to return to the vaporizer by gravity, it takes place at adistance from the point of'entrainment and has no function in preventing the backward flow 'of vapor from the entrainment region to the vacuum space, said function being pro-' 'vided by the parallel or uni-directional character of the jet. With certain forms of parallel jet, the introduction of a confining or containing space in the entrainment and compression regions is totally unnecessary (see Fig. 7). :In the condensation pump the et has a certain degree of kinetic character in overcoming the back pressure, but the necessity of local cooling at the point of entrainment to prevent the flow of a dense vapor stream into the vacuum space shows that it has not the para el character of my In the so-called difiusion pump, the vapor stream exerts a substantiallyuniform pressure in all directions at the oint of entrainment, and this pressure is i her than the back pressure. The stream the character of a kinetic jet. With e ual opportunity the vapor would tend to owequally into the vacuum and dischar spaces, and it is necessary to provide a su stantially continuous wall to confine the stream. The vapor enters thestream by passing through a minute orifice in this wall, a henomenon analo us to the well known ifiusion of t rough a porous membrane. M 1 method is fundamentally distinct from't at of the diffusion pump as vwell as the condensation pump inthe selfcontinent characterof the parallel free jet which I em loy.

- annular form.

For an i lustration of my method and- .some of the forms m apparatus may take,

- on the line 1010 of Fig. 9.

Fig. 11 is a section taken at the line 11-11 on .Fig. 2 and viewed in the direction of the arrows. v

Referring to Fig. 1, B is a boiler of glass,

" metal or other suitable material, in which is 05 which the chamber A communicates through a mass of mercury or other suitable material Hg which is adapted to be va rized by the application of heat, as by a unsen burner, electric heater or otherwise, to the bottom of the boiler Leading from the boiler B is the vapor conducting tube T communicating with the expansion nozzle N having the restricted throat t, the cross sectional area of the passage within the nozzle N increasing to the orifice o. Surrounding the nozzle N is the'chamberA with which communicates the tube (3' communicating with the X-ray tube, vacuum tube or other chamber fromwhich gas is to be removed,

to cause therein a 'very'low pressure. or high vacuum. VVith the bottom of the chamber communicates'a tube a communicating with the bottomof the boiler B to. return thereto the condensed mercury or other material employed. Dv is a second chamber with oes not have the compression passage E, a tube 12 leading from the bottom of the chamber D into the chamber A. The tube 0, laterally deflected within the chamber-D, communicates with a reliminary or rough'vacuum pump.

bottom, may be covered with lagging or material of poor heat conductivity, indicated at d, which may also extend upwardly around the tube T. Or any other suitable means may be employed for preventing heating of the apparatus above the boiler B, though such provision is not necessary and my invention is not limited thereto.

The preliminary or rough vacuum pump maintains within the chamber D a pressure he boiler B, except at its heat receiving of from .02 to 1 millimeter of mercury,

more or less.

The operation is as follows: I Heat isapplied to the bottom of the boiler B at such rate that the pressure within the boiler amounts to from 10 to 100 through the tube T through the throat t and is expanded through the nozzle N, attaining at the nozzle orifice an exceedingly high ve oo ity estimated as from 20,000 to 50,000

centimeters per second, and a temperature much lower than the boiler temperature. This insures a parallel jet of vapor which is directed into the compression passage E, a portion of the vapor striking the walls thereof at an, acute angle. The gas to heremoved passes from thevessel to be evacuated through the tube G into the chamber A and thence by molecular =flow'into the vapor jet between the nozzle N and passage E. The gas enters a great distance into the vapor jet before collision with the vapor molecules, and the mixture of gas and vapor is delivered at relatively far higher pressure into the chamber D, wherein a separation between the vapor and the gas takes place,

because the vapor is condensed, particularly at the upper end of the chamber D, while the gas passes out throughthe tube a to the preliminary or rough vacuum pump, which then'again steps the pressure up to atmospheric. The vapor condensing within the chamber D again becomes liquid andv collects at thebottom of the chamber D and flows through the trap or siphon tube b and drops'through the chamber A to its bottom, from which it is delivered'by tube a back into the boiler B. The lateral deflection of the tube 0 within the chamber D largely prevents vapor from entering the tube.

In the compression passage E the volume of the vaporis again reduced after expanorificeo' of the nozzle N and the point of incidence of the vapor upon the inner wall of the passage E the vapbr jet maintains its form as a free jet, without material fringing or dispersion. I

he high velocity and resultant high kinetic energy of the vapor enables a small quantity of vapor to overcome the back pressure, which is of decided advantage in enabling operation with a highly rarefied jet into which the gas will penetrate readily.

The jet in this pump is invisible, indicating thatthe vapor in the jet does not condense, although cooled by expansion considerably below the saturation point. attribute this to the short time and few collisions in the jet and the absence of condensation nuclei. The vapor is hence in a super-saturated condition. This is not an essential feature of my method, however,

which would operate equally well if a considerable portion of the vapor condensed within the jet in the form of minute droppartially reconverted into pressure of the vapor and partially lost inoVerc'oming friction, the vapor and gas entering the cham ber D at reduced velocity but higher pressure.

So far as I am aware, I am the first to employ the relatively high boiler pressures above indicated, these boiler pressures being of a different order of magnitude, and .I believe from to 100 times higher than heretofore employed. The resultant velocity of the v'apor jet is also of a different order of magnitude from what has hereto- I fore been employed, though the exact ratio lets, which would entrain and expel the gasin a manner similar to the molecules of the vapor. Such condensation could be provided, for example, by atomizing liquid mercury into the jet at the throat t.

It is the uncertainty as to the exact condition of the vapor jet as regards condensation of liquid drops within the jet that has,

led me to lay little emphasis on numerical values of velocities in describing my invention. The assumption that the vapor expands adiabatic-ally like an ideal gas leads to one set of velocities, temperatures and densities. The assumption that it expands as a perfect vapor leads .to velocities approximately twice as high corresponding to about four times as much energy, said increased supply of ener being theoretically attained by the liberation of latent heat due to condensation within the jet, as is well known in steam pracice. The second assumption further leads to higher final temperature, and consequently greater relative molecular velocities, but since the translational velocity is also much greater, the relation essential to parallel flow remains. That is, the existence of the free parallel jet does not prove or distprove either theory, but the transparency o the jet favors the first. Consequently, I prefer to assign somewhat is diflicult to estimate. The essential -difl'.erence consists in the combination of high velocity, cooling due to expansion, and small angle of divergence of .the free jet in my pumps, all of which are of a different order from those in previous pumps of the high vacuum class.

It is found that thecompression passage E or the upper end of the chamber A, or both, may be heated from the exterior with out materially afiecting the behavior and efficiency of the pump. ,.And in no case is'it necessary to cool thes'ej'parts, or either of them, or the parts within which is the region in which the gas enters the vapor. Even if the compression passage is cooled, it does not, as is the case in acondensation pump, cause most of the mercury to condense on the walls near the nozzle. In the pump of Fig. 1, cooling the compression passage E by ice did not seriously impair the parallel character of the jet, and the greater portion of the vapor still condensed in the chamber D, which remainedat about 60 degrees C.

Without limiting my invention thereto, it may be stated, by way of example, that suit 105 able proportions for a conical nozzle to produce a jet of the character used in my inven-' tion in the form of pump shown in Fig. 1," are as follows: Minimumor throat diameter, .040 inch; diameter of. mouth, .25 inch;

' length from throat to mouth, .6 inch diamarbitrarily for purposes of discussion ve- 55 locities between the limits of 20,000 and 60000 centimeters per second to the jets which I emplo existing evidence j ustifying' a belief that t 'e actual velocities lie within this range,

The compression passage E is referably short and of large diameter, the/ ength be- -ing preferably less than twice'the diameter to minimize t e frictional effects which are very large at such low pressures' In the 65 passage E the kinetic energy of the jet is eter of compression passa (cylindrical) 1 inch; length, 2 inches. v iiah aboiler pressure of about 35 millimeters, the jet produced 'by{ this nozzle was operative in producing vacua' of the order of .00004 millimeter of mercury againsta back pressure of about .03 millimeter, oduced by the preliminary pump, and with a boiler pressure of 100 millimeters against a pressure of .1 millimeter or more. The molecular density at .lmillimeter pressure is about 5X10 showing that the vapor was compressedto approximately 2.5 times its lowest density of 2 l0 -f. v By the operation described, practically any, degree ofvacuum may be produced in the vessel communicating withthe tube C. The speed of pumps of this type, when properly constructed in accordanoewith the prin- 130 ciples described, may be expressed in Gaede units of the order of 1000 or more cubic centimeters per second.

In Fig. 2, the boiler B communicates with the upwardly extending tube or chamber A trally sup orted b the wire or other suitable mem r 2', an open at its lower end, and suitably spaced by the .-wire spacer h from the upper end of the tube T, the space between the upper end of the tube T and the lower end of the member N constitut ing the expansion nozzle N. The supporting-member z' flares upwardly and so main tains itself in the bottom of the tube C--- which communicates with the vessel to be evacuated. The tube a communicates with the interior of the tube A and with the preliminary or rough vacuum pump.

In this case the mercury or other vapor 'pas'ses upwardly through the tube T into the member N and thence downwardly through the nozzle N, in which it is expanded, and the jet is directed into the compression passage E from which the mixture of the gas, entering through the tube C and entering into the jet below the nozzle N, is discharged at lower velocity and higher pressure into the tube A, in which the mercury or other vapor condenses and returns to the boiler B, while the gas passes out through the tube a to the preliminary or In Fig. 3' the structure is similar to that shown in Fig. 1, except that the nozzle N is rough vacum pump.

supplanted by a closure at the upper end of the tube T provided with a small aperture 9' through which the vapor is expanded. A diaphra is having theaperture m is positioned a ve the orifice jand serves to allow passage of the free vapor jet andintercepts that part'of the vapor which issues from the orifice j and tends scatter or depart from the free jet, the'intercepted vapor'oondensr ing chiefly on the walls of A, and passing back to the boilerthrough the tube a.

Above the diaphragm is is the compression passage E above which is the chamber D with which communicates the tube a leading to the'preliminary or rough'vacuum pump. The vapor finally condensing in the chamber D returns as liquid through the tube 6 to.

the upper side of the diaphragm is which disposed a diaphragm is such as described in connection with Fig. 3, the diaphraginlc in both cases serving to maintain a more perfect free'jet.

In Fig. 5 the structure is similar to that of Fig. 1, except that the tube T extends laterally or horizontally, as do also the nozzle N and the compression passage E, which latter in this instance is shown as convergent and then d1vergent, and the nozzle is curved to direct the jet more accurately than in Fig. 1.

In Fig. 6 the tube T extends laterally or horizontally and communicates with the expansion nozzle N disposed within the chamber I) through a wall of which extends the tube C communicating with the vessel to be evacuated, the tube c communicating with the preliminary or rough vacuum pump.

The lower end of the tube C is closed at p, and a lateral wall of the tube C forms the diaphragm' is having the orifice m, as in Figs. 3 and 4. The vapor expanded by the nozzle N passes through the orifice m and thence-through the com ression passage E and is dischargedinto tlie tube D, the gas from the tube C entering the vapor after it has passed. the orifice m and before it enters the passage E. In this figure, the jet must have a less parallel character when issuing from the nozzle N, this condition being at tained by using a smaller ratio of areas of mouth to throat of the nozzle than in 1 and 5, in order that the fringe of the jet between N and we will be sufiicientlydense to prevent gas from D entering the jet and so being carried into C. The true parallel character, and the entrainment, take place from m to E.

In. Fig. 7 the tube T communicatin with the boiler B communicates with .a cyindrical chamber 9 through which extends the' tube C, which communicates with' the vessel. to be evacuated, the tube C terminating in a somewhat tapered part r. Around the part 1' is the expansion nozzle N, of ring form, communicating with the. chamber 9.

The vapor delivered from the expansion nozzle N into the chamber D forms a hollow jet into which is drawn the gas through the tube C, the mixture being delivered into the Chamber- 1). .In this arrangement. a compression'passage is dispensed with. but there is in the interior (if the chamber D a fairly definite compression region wherein 'the kinetic energy of the jet is reconverted into pressure." The only barrier against the relativelly higher pressure of gas from the chamber returning into the tube C is the vapor jet itself.

In F ig. 8 is shown a multi-stage type of pump, wherein the vapor generated the boiler B rises in the tube T on whose upper end rests the sleeve or tube 8, Fig. 9, on whose upper end rests the tube t having the lateral openings f and on whose upper end rests the member t having the lateral openings t and upon whose upper end rests the tube t having the lateral openings t and on whose upper end rests the closing cap at through which extends the bolt y having at its lower end an eye g embracing the pin y sealed in and extending across the bore of the tube T, a nut 3 serving to force the cap a: downwardl andvso hold the parts in firm assembly. T e downwardly extending nozzle member 2 is supported upon the member t, the second nozzle forming member 2 1s supported upon the member t and the third nozzle forming member 2 is supported upon the member t.

Vapor passes upwardly through the tube T and thence through the passages t and thence-through the narrow passage or throat formed between the inner wall of the flaring member .2 and the upper end of the member 8 through the expansion nozzle N intothe tube F, which delivers into the chamber D with which communicates the tube 0 leading to the preliminary or rough vacuum ump.

through the passages t through the throat 'formed between the flaring nozzle member 2 and the upper end of the member z, is expanded in the nozzle N and delivered into the tube F; and vapor passes also from the tube T through the passages t through the third expansion nozzle N into the tube F.

The gas from the tube C enters the jet issuing from the nozzle N by passing around the lower edge of the member 2; the mixture of the vapor from the nozzle N and the gas then passes downwardly past the lower edge of the member .2 and is carried along by the vapor lssuing from the nozzle 1' N past the lower edge of the member 2, and vapor and gas are carried by the vapor issuing from the nozzle N into the tube F and thence into the chamber D where the vapor condenses and returns to the boiler B through the tube a.

The nozzle N delivers more vapor than the nozzle N, which in turn delivers more vapor than the nozzle N. Of these multiple nozzles, the uppermost, N, and its associated structure must be so proportioned and constructed as to produce high vacuum by producmg aparallel or uni-directlonal jet. The

I areas for the entry of the gas, and mixtures of gas! and vapor, become progressively smaller, thearea of the passage around the apor passes also from the tube nozzle member a being greater than that around the nozzle member a, which in turn is larger than the area of the passage around the lower edge of the member 2.

The successive stages raise the ressure of the gas entering from the tube in successive steps until the relatively high pres sure in the chamber D is attained.

As seen in Figs. 8 and 10, the nozzle forming member 2 may be corrugated, as indicated at a, in order to impart to the jet issuing from the nozzleiN 'a greater superficial area.

While I have described my apparatus as operating against pressures produced by the preliminary vacuum pump which are hlgher than the pressure in the vapor stream at the point of entrainment, my method and apparatus are equally operative if it happens that the preliminary pump is able to produce a pressure lower than the pressure in the vapor stream at the point of entrainment, and my inventionis not limited to either condition.

It is further to be noted that my method and apparatus will remove from the vacuum space vapors as well as fixed gases.

While the hereinbefore described forms.

are suitable for carrying out my method, it will be understood that the apparatus may be used to produce dlflerent boiler pressures, jet velocities and vapor densities than herein described in connecof apparatus tion with my method, andother apparatusv -may be used for operating in accordance with the method described, without departin from my invention.

What I claim is:

1. The method of producing a, high vacuum which consists in generating an actuating vapor, delivering-said vapor jby expansion in a free jet in which the vapor is expanded to such a degree that the velocity with the relative velocities of the molecules therein that an approximately parallel-molecular flow of vapor is produced in the jet, and admitting elastic fluid from the space to beexhausted into said jet. t

2. The method of producing a high vacuum, which consists in generating an actuating vapor, delivering said vapor in a free jet in which the vapor is expanded to such a'degree that a special absorptive con,- dition of the jet is produced, whereby said jet is enabled to entrain elastic fluid whose pressure is substantially less than the pressure in the jet and admitting elasticafluid from the space to be exhausted into said jet.

3. The method of producing vacuum higher than one-tenth of a millimeter of mercury, which consists in generating a motive vapor, as of mercury, having a pressure upward of two millimeters of mercury, expanding the motive vapor in a nozzle-to of the jet is so high in' comparison iao produce a free jet in which the vapor pressure is of the order of one-tenth to one-hundreth of a millimeter of mercury, whereby a parallel jet is produced in whlch the individual velocities of the vapor molecules, relative to a molecule heaving the jet velocity, are small compared with the jet velocity, en training the elastic fluid from the space to be evacuated by said motive fluid jet, and

increasing the pressure of the mixture to a pressure below atmosphericby converting the kinetic energy of the mixture intopressure.

4. The method of producing vacuum higher-than one-tenth of a millimeter of mercury, which consists in generating a motive vapor, as of mercury, having a pressure in excess of two millimeters of mercury and less than atmospheric pressure, expanding said motive vapor in a nozzle to produce a free jet whose molecular density lies between 10 andlO gram molecules per cubic centimeter, entraining elastic fluid from the space to be evacuated by said m0- tive jet,'-and increasing the pressure of the mixture to a pressure below atmospheric by converting the kinetic energy of the mixture into pressure.

5. he method of producin vacuum higher thanone-tenth of a mil imeter of mercury, which consists in generating a motive vapor, as of mercury, having a pressure in excess of two millimeters of mercury and less than atmospheric pressure, expanding said motive vapor in a nozzle to produce a free jet whose molecular density lies between- 10- and 1O gram molecules per cubic centimeter, entraining elastic fluid from thespace to be evacuated by said motive jet, in-

creasing the pressure of the mixture to a pressure belowatmospheric by converting the kinetlc energy of the mixture into presmemes sure, condensing the motive vapor and sepa rating it from the compressed elastic fluid, and thereafter raising the compressed elastic fluid to higher pressure.

6. The method of producing a high vacuum which consists in generating an actuating vapor, delivering said vapor by expansion in a free jet in which the vapor is expanded to such a degree that the scat tering; of the jet when surrounded by a high vacuum is substantially nullified, and admitting". elastic fluid from the space to be exhausted into said jet.

7. A vapor pump for producing high vacuum, comprising a boiler, an expansion nozzle having a throat and a nozzle passage increasing in crosssection from said .throat,

a connection from said boiler to said nozzle; a connectlon from the vessel to be evacuated to the space surrounding the jet issuing from said nozzle, a compression passage, a condensing chamber communicating with said compression passage, and a drain passage from said condensing chamber to sald boiler.

8. A vapor pump for producing high vacuum comprising a boiler for producing motive vapor means for expanding said vapor to pro uce a free jet, a. condensing chamber through which said jet is directed, a compression passage into which said jet is delivered beyond said condensing chamher, a connection from the space to 'be exhausted to the space between said condensing chamber and said compression passage, and a second condensing chamber beyond said compression passage.

In testimony whereof I have hereunto affixf gl my signature this 5th day of January, 19 4.

WILLIAM W. CRAWFORD. 

